In LTE networks, downlink control information (DCI) is transmitted to user equipment (UE) in the Physical Downlink Control Channel (PDCCH). The LTE UE obtains the resource allocations for uplink and downlink transmissions from the PDCCH. The PDCCH onto which the DCI is mapped has different formats and depending on its size is transmitted in one or more control channel elements (CCE). A CCE corresponds to thirty-six resource elements (REs).
In LTE Release 8-10, legacy PDCCH is transmitted and demodulated based on cell-specific reference signals (CRS). A legacy PDCCH is precoded with transmit diversity on 1/2/4 CRS antenna ports, cross-interleaved with other PDCCHs, and then distributed over the entire system bandwidth in the legacy control region of a subframe. The legacy control region comprises of the first K orthogonal frequency division multiplexing (OFDM) symbols in the first slot of a subframe, where K=1, 2, 3 for system bandwidths greater than 10 Physical Resource Blocks (PRBs), and K=2, 3, 4 otherwise. The control region size K is signaled in the Physical Control Format Indicator Channel (PCFICH). Through CRS-based transmit diversity and cross-interleaving, legacy PDCCH exploits spatial and frequency diversity to achieve robustness and ensures its reliable reception and cell coverage.
In LTE Release 10, a new PDCCH transmission scheme is introduced for the relay backhaul link, called R-PDCCH. R-PDCCH inherits all the DCI formats of legacy LTE systems (i.e., DCI formats 0, 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 4). R-PDCCH is transmitted in the Physical Downlink Shared Channel (PDSCH) region to exploit higher control channel capacity. R-PDCCH can be transmitted with both CRS-based and Demodulation Reference Signal (DMRS)-based transmission. For DMRS-based R-PDCCH, transmission is based on single-layer beamforming on DMRS antenna port 7 with a scrambling ID (SCID) of 0. R-PDCCH is transmitted in 1/2/4/8 Virtual Resource Blocks (VRBs), which are dynamically selected from a subset of NVRBR-PDCCH VRBs semi-statically configured by a higher layer.
An enhanced PDCCH (EPDCCH) is introduced in LTE Release 11 to expand on R-PDCCH. Similar to R-PDCCH, the EPDCCH is transmitted in the PDSCH region to achieve higher control channel capacity. The higher layer configures a set of PRBs as the EPDCCH resources. The PRBs provide a physical grouping of subcarriers and OFDM symbols. The EPDCCH reuses the concept of CCEs to map the EPDCCH onto the PRBs configured by the higher layer for EPDCCH transmission. Demodulation of EPDCCH is based on DMRS transmitted in the PRBs that are used for transmission of the enhanced control channel. EPDCCH is not demodulated by CRS such that it can be transmitted in non-backward-compatible carriers and Multicast-Broadcast Single Frequency Network (MBSFN) subframes. Antenna ports 7-10 or subsets thereof are used with EPDCCH.